Development of digital technologies for recording and playing back images and sound, and for transmitting these images or sound in a free space or over a cable, is now reaching a point at which it is evident that they offer significant advantages in comparison with conventional analogue techniques. Vision and sound quality, spectrum and power efficiency, service flexibility, multimedia convergence and potentially lower equipment costs are some of the advantages. Use of digitized signals for delivery of video and audio services to individual subscribers is continually growing, and has already become a dominant form of distribution in many parts of the world.
Digital systems for broadcasting, receiving, recording, or playing back digital video or audio data frequently use an error correction technique called forward error correction (FEC). In an FEC technique, a redundancy information is encoded into digital data according to a pre-defined algorithm, so that data errors can be corrected upon reception, without having to request a repeated transmission or playback. Such a request may be impractical or impossible to fulfill in a digital one-way broadcasting system or in a digital playback system.
It is common for the encoded symbols of the data stream to be purposely shuffled, or interleaved, using a pre-determined interleaving pattern. On the receiving end, the symbols are de-shuffled, or “deinterleaved” using the same pattern in reverse, and then decoded to recover information lost due to errored symbols. The interleaving technique is utilized to protect a digital system against so called burst errors, or errors caused by a sudden burst of noise impacting two or more neighboring symbols in a digital data stream. Without it, an FEC decoder might not be able to recover all the lost information caused by the noise burst due to, for example, a scratch on a DVD being played, or due to an electromagnetic pulse induced in a coaxial community antenna television (CATV) cable plant. A comparison can be drawn to a case of comprehending a corrupted printed word in a sentence. When one letter of the word is corrupted or missing, the entire word, in most cases, can be comprehended. When, however, two or more letters are missing in the word, the entire word may become incomprehensible to a reader.
Even though the interleaving technique is successfully used for improving the level of tolerance of a digital communication link to burst noise, it has limitations related to the circuit complexity, amount of memory required for interleaving and deinterleaving, as well as to a latency of signal reception due to interleaving and deinterleaving. It is prudent to implement just as much interleaving as required to effectively suppress a bit error rate (BER) increase due to a burst noise that is typical for the system in question. However, in modern digital systems, the BER depends on a large number of factors, including burst and random noise, signal strength, modulation type, encoding parameters selection, and so on. As a result, the contribution of burst noise to BER cannot be easily deduced. Furthermore, the burst noise can change with time, which may require a periodic re-optimization of the system parameters. A device and method for determining a contribution of burst noise to a BER in a digital system is therefore highly desirable.
Typical prior-art test instruments for measurement of the BER of a CATV quadrature amplitude modulated (QAM) signal use information from a Reed-Solomon (RS) decoder to calculate the so called pre- and post-FEC BER. There is no distinction made between bit errors caused by random noise and bit errors caused by burst noise.
In U.S. Pat. No. 6,662,332 by Kimmitt, which is incorporated herein by reference, a method and apparatus for detecting errors in serially transmitted digital data is taught, in which encoding and interleaving is employed. In one embodiment, Kimmitt teaches increasing interleaving depth when detected BER exceeds a threshold value. However, the apparatus of Kimmitt does not discriminate between BER increase caused by random and burst errors. Consequently, when the errors are random and not burst errors, increasing interleaving depth as described by Kimmitt does not result in BER reduction.
In U.S. Pat. No. 5,942,003 by Ivry, which is incorporated herein by reference, a burst error detector for lowering a receiver BER is taught. In a detector of Ivry, a deviation of detected symbol complex amplitude from a value corresponding to a recognizable symbol is measured at a demodulator stage of the receiver. Based on the measured magnitude of the deviation, an error burst detection circuit evaluates probability that a symbol is received in error due to burst noise. This information, together with location of the errored bits in the data stream received, is sent through a de-interleaver to a block RS decoder. The RS decoder uses this additional information to increase the number of detectable and correctable bits. The apparatus of Ivry is capable of effectively doubling the number of correctable symbols as compared to a conventional RS decoding based FEC technique. Disadvantageously, the apparatus of Ivry is considerably more complex than a conventional RS decoder, and requires fine tuning of the demodulator electronics. In addition, the apparatus of Ivry is constructed to detect a single symbol error, not a burst of errors.
The methods of the prior art either do not distinguish between random and burst noise, or require complex electronics to detect reception errors caused specifically by burst noise. Accordingly, it is a goal of the present invention to provide a method and a simple system for determining a contribution of burst noise and random noise to the errors of reception of a digital data stream.